Heat exchanger

ABSTRACT

A plate-fin type of heat exchanger (100) facilitates exchange of heat between two or more process streams (101, 102). It comprises a matrix (M) of two different types of heat exchange plate elements (P1, P2) inter-digitated with each other. Adjacent plate elements are metallurgically bonded together for good thermal contact by an activated diffusion bonding process. The plate elements (P1, P2) are high-integrity diffusion bonded sandwich constructions comprising two outer sheets (201, 203 - FIG. 4 and a superplastically expanded core sheet structure (202) between the two outer sheets. The sandwich construction provides flow passages (P) for the process streams. Adjacent plate elements (P1, P2) carry different process streams (101, 102).

This invention relates to heat exchangers of the kind generally known asplate-fin heat exchangers are

The fluid passages in plate-fin heat exchangers are defined bypartitions of a metal which has a satisfactorily high coefficient ofheat transfer, so that when a high temperature fluid is passed throughsome passages and low temperature fluid is passed through furtherpassages which are adjacent thereto, there results a cooling of theoriginally high temperature fluid, by heat conduction through thethickness of the partitions into the cool fluid. Efficiency of heatexchange is boosted by inclusion in the fluid flow passages of so-called"fins", which may in fact be corrugated members, dimples, grooves,protuberances, baffles or other turbulence promoters, instead of fins assuch.

Plate-fin heat exchangers offer significant advantages over shell-tubeheat exchangers in terms of weight, space, thermal efficiency and theability to handle several process streams --i.e. several streams of heatexchange media--at once. However, most current plate-fin heat exchangertechnology is centred on a brazed matrix construction using aluminiumcomponents and is therefore limited to low pressure and low temperatureoperation. Even using other materials, such as stainless steel,operational pressure limits (say, 80-90 bar) apply because of the use ofbrazing as the method of fabrication.

Our prior patent applications EP90308923.3 and GB9012618.6 disclosealternative ways of manufacturing plate-fin heat exchanger elementswhich help to avoid the above problems and allow greater flexibility intheir design. Among other things, they describe a method ofmanufacturing heat exchange plate elements in which metal (e.g. titaniumor stainless steel) sheets are stacked together and selectively.diffusion bonded to each other before being superplastically deformedto a final hollow shape defining internal passages, which canincorporate integrally formed "fins". Use of superplastic deformation inthe manufacturing process enables the generation of high volumefractions of hollowness in a heat exchanger element. For example, iftitanium sheets are used as the starting point, the result is a highintegrity, low weight heat exchanger element which can operate atinternal pressures in excess of 200 bar and at temperatures up to 300°C. Stainless steel elements will operate at higher temperatures andpressures.

One object of the present invention is to facilitate easy manufactureand assembly of heat exchangers incorporating matrices of suchsuperplastically formed/diffusion bonded heat exchanger plate elements.

A further object is to provide very high integrity matrices of suchplate elements.

According to the present invention, a plate-fin type of heat exchangerfor facilitating exchange of heat between at least two process streams,comprises;

a matrix of heat exchange plate elements arranged in side-by-side heatexchange relationship, the plate elements comprising diffusion bondedsandwich constructions, each such sandwich construction having two outersheets and a superplastically expanded core sheet structure between thetwo outer sheets, each core sheet structure providing flow passage meansfor at least one process stream, adjacent plate elements being inintimate thermal contact with each other over at least most of the areasof their side faces through bonded joints between them, and

process stream inlet and outlet manifold means integral with the matrixfor passing the process streams through the plate elements, the manifoldmeans penetrating the matrix from side-to-side through the thicknessesof the plate elements.

Preferably, for maximum strength and heat and corrosion resistance ofthe heat exchanger matrix, the bonded joints between adjacent plateelements are metallurgically bonded joints, especially diffusion bondedor activated diffusion bonded joints. If activated diffusion bondedjoints are utilised, they are preferably protected from contact withaggressive process stream fluid in the manifold means by autogenous sealwelds spanning the joints between the penetrated plate elements.

Further aspects of the invention will be apparent from a reading of thefollowing description and claims.

An exemplary embodiment of the present invention will now be describedwith reference to the accompanying drawings, in which:

FIG. 1 is a part-sectional view of a complete heat exchanger accordingto the invention;

FIGS. 2A to 2C illustrate a process for manufacturing a heat exchangerplate element suitable for use in the present invention;

FIGS. 3 is a plan view of a heat exchanger plate element suitable foruse in the present invention, its top face being removed to show itsinterior structure; and

FIG. 4 is a perspective detail view of that part of the heat exchangerplate element in FIG. 3 which is indicated by arrow IV.

Superplastic forming, diffusion bonding and activated diffusion bondingare well known metallurgical phenomena.

Superplasticity is a deformation phenomenon which allows some materialsto strain by large amounts without the initiation of tensile instabilityor necking. This enables the generation of high volume fractions ofhollowness in a heat exchanger matrix, while allowing designs of goodmechanical and thermal performance, together with low weight and highutilisation of material.

Diffusion bonding is a solid state metal interface phenomenon in which,provided clean metal surfaces at a suitable temperature are protectedfrom surface contamination by the provision of a suitable joint faceenvironment, and sufficient pressure is applied to the mating surfaces,then solid state diffusion of the metal atoms across the boundary takesplace to such an extent that subsequently no interface can be detected.No macroscopic deformation takes place during bonding and thereforeshape and size stability is maintained during the operation.Furthermore, the joint produced has parent metal properties without thepresence of a heat affected zone or other material such as a flux orbond promoter. Its use within a heat exchanger therefore reduces thepossibility of chemical interaction with process fluids.

Activated diffusion bonding differs from diffusion bonding in that thefaces of the metal components to be joined are coated with an activatorwhich, at the temperatures and pressures use to achieve the joint,becomes liquid and promotes diffusion of atoms across the interfacebetween the components. The activator is a metal alloy of lower meltingpoint than the metal of which the components are made, butmetallurgically related thereto. As a consequence of the differingmetallurgical composition of the joint relative to the parent metal oneach side, activated diffusion bonded joints, unlike solid statediffusion bonded joints, do not exhibit parent metal properties withrespect to stress and corrosion resistance.

Referring to FIG. 1, there is shown a plate- fin type of heat exchanger100 for facilitating exchange of heat between two counterflowing processstreams, 101,102. The heat exchanger matrix M comprises a stack of twotypes of plate elements P1, P2 which are inter-digitated with each otherand whose side faces are metallurgically bonded to each other so thatthey are in intimate thermal contact with each other over at least mostof the areas of their side faces through the metallurgically bondedjoints between them. Intimate thermal contact may be defined as thatcontact which ensures substantially unhindered flow of heat betweenadjacent heat exchange elements, i.e. , compared with the material ofwhich the elements are made, thermal conductivity does not reducesignificantly at the interfaces between the elements.

For reasons of structural strength and integrity in the heat exchangermatrix, we have chosen in the present embodiment to achieve thenecessary intimate thermal contact between adjacent heat exchangeelements by means of metallurgically bonded joints, specificallydiffusion bonded joints. Nevertheless, it would alternatively bepossible to utilise other suitable bonding means, such as brazing, toachieve intimate thermal contact between the elements, provided that thematrix structure so achieved was sufficiently strong, with sufficientheat and corrosion resistance, to be useful for the duty envisaged.Here, a bonding means capable of achieving intimate thermal contact canbe defined as a good thermal conductor which, when introduced betweenthe elements under appropriate manufacturing conditions, obviates thesurface asperities of the surfaces to be brought into thermal contactwith each other.

Plate elements P1 are intended to have process stream 101 flowingthrough them and plate elements P2 are intended to have process stream102 flowing through them. Whereas the plate elements P1,P2, etc., in themiddle of the matrix stack M are all of the same gauge of titanium alloyin the present example, the front and back end elements of the matrixare manufactured with a thicker sheet on one side to form side plates107 to which nozzles and supports may be welded.

The heat exchanger matrix M is provided with inlet and outlet manifoldsIM1,OM1, IM2,OM2 for supplying the plate elements P1, P2 with theprocess streams 101,102 respectively. The manifolds are integral withthe matrix, and the plate elements constituting it, and penetrate itfrom side-to-side through the thicknesses of the plate elements. Supplypipes SP1,SP2 and outlet pipes OP1,OP2 carry the process streams 101,102to and from the heat exchanger. Because the end elements of the matrix Mare manufactured with relatively thick outer sheets forming the sideplates 107, these pipes can be securely fixed to the heat exchangerthrough the hemispherical supports 109, which are welded to the sideplates 107.

Although hemispherical supports 109 are shown in FIG. 1 as supports forthe pipes, they are not invariably a necessary part of the constructionin most cases, the ends of the pipes or nozzles OP1, OP2, SP1, SP2 canbe welded directly to the side plates 107.

In the present embodiment the plate elements P1,P2 are ofsuperplastically formable titanium alloy, but other superplasticallyformable materials such as stainless steel and aluminium alloys may beused, depending on the duty for which the heat exchanger is intended.

The plate elements P1,P2 comprise diffusion bonded sandwichconstructions, each such sandwich construction having two outer sheetsand a superplastically expanded core sheet structure between the twoouter sheets. This construction of the plate elements will now befurther described with reference to FIGS. 2A to 2C and 3 as well as FIG.1.

The heat exchanger plate elements are manufactured by a superplasticforming/diffusion bonding process which will first be briefly describedin a simplified manner with reference to FIG. 2. For fuller details ofmanufacture, reference should be made to our earlier patent applicationsEP90308923.3 and GB9012618.6.

Referring to FIG. 2A, three superplastically formable metal sheets201,202,203 (made of, say, a suitable titanium alloy), of near net shapeand controlled surface finish, are cleaned to a high standard and a bondinhibitor is deposited onto selected areas of the joint faces F1, F2 ofthe two outer sheets 201,203. Within boundary B, white areas indicatewhere the bond inhibitor is deposited, but outside boundary B, no bondinhibitor is deposited. The deposit specifies the ultimate internalconfiguration of the finished heat exchanger plate element, andcomprises areas defining process stream inlets I and outlets 0, inletand outlet flow distributor regions DI and DO respectively, and flowpassages P within the element. Edge regions E of the sheets 201,203,where it is not desired to produce an internal structure, do not haveany bond inhibitor applied.

Although the internal geometry is fixed at this stage, the depositionprocess, e.g. silk screen printing, allows considerable flexibility ofdesign to satisfy both mechanical and thermal requirements.

The sheets 201,202,203 are then stacked and diffusion bonded together inthe manner detailed in our earlier patent applications, resulting in abonded stack 205, which is placed in a closed die D as shownschematically in cross-section in FIG. 2B. However, where bond inhibitorhas been applied in areas 206, diffusion bonding has not taken place.

Superplastic forming of the bonded stack 205 into an article which isalmost the final shape of the heat exchanger plate element, completewith its internal structure as shown schematically in FIG. 2C, nowoccurs.

The bonded stack 205 and the die D are heated to superplastic formingtemperature and the stack's interior structure, as defined by thepattern of bond inhibitor, is injected with inert gas at high pressureto inflate the stack so that the outer sheets 201,203 move apart againstthe die forms. As the outer sheet 201 expands superplastically into thedie cavity, it pulls the middle or core sheet 202 with it wherediffusion bonding has occurred. Superplastic deformation of the coresheet 202 therefore also occurs to form a hollow interior which ispartitioned by the stretched portions 207 of the core sheet 202, therebycreating passages P through which process stream can flow. The edgeregions E of the stack 205 remain fully bonded, and therefore flat andunexpanded.

It is convenient for manufacturing purposes if all the sheets201,202,203 are made of superplastically formable titanium alloy, orother superplastically formable metallic material, though only thesheets 201 and 202 are in fact superplastically formed duringmanufacture of the element.

After the superplastic forming process has been finished, each articleso produced is trimmed around its edges and the manifold holes,indicated by the circles in FIG. 2A, are drilled. When the manifoldholes are drilled, they create circular slot openings into those partsof the expanded internal structure which define the inlet I and outlet0. After drilling, the inlet slot I and the outlet slot O are, for thepurposes of the present embodiment, completely opened up internally forflow of a single stream of the process fluid by a machining operation tocut away obscuring portions of the core sheet 202. This produces theheat exchanger plate element P1 as further illustrated in FIG. 3, whichis ready for incorporation in a matrix of such elements by a diffusionbonding process as mentioned previously.

The plate element shown being produced in FIG. 2 is in fact one of theelements P1 shown in FIG. 1. The other elements P2 are similar to theelements P1 except that their internal core sheet structures areslightly differently arranged for connection of their inlets and outletsto their respective manifolds IM2, OM2. The internal cavities formed inthe plate elements P1, P2 during the superplastic forming process areasymmetrically shaped so that the manifold holes for the stream whichdoes not enter the element are drilled though the solid metal formed bydiffusion bonding of the edge portions of the sheets. Thus, in FIG. 1,the manifold hole IM1 connects process stream 101 to plate element P1,but not to the immediately preceding and succeeding plate elements P2 inthe stack, whereas manifold hole IM2 connects process stream 102 toplate elements P2, but not to plate elements P1.

We suggest the activated diffusion bonding process is used to make theheat exchanger matrix from the plate elements, rather than attempting tosolid state diffusion bond adjacent plate elements together in the sameway as was done during the manufacture of the plate elements themselves,because of the danger of the individual hollow plate elements collapsingunder the higher temperatures and pressures necessary for solid statediffusion bonding without an activator. However, if such collapsing ofthe elements is not a problem in a specific matrix design, or can beotherwise obviated, it is preferable to utilise solid state diffusionbonding of the plate elements into the matrix, so as to avoidmetallurgical differentiation at the bond line, with its attendantcorrosion risks if the joint is exposed to a chemically aggressiveliquids or gases.

The superplastic forming/diffusion bonding process outlined aboveresults in the production of very accurately formed external surfacesfor sheets 201,203, which enable good conformance of each heat exchangerelement to its neighhours in a matrix of such elements.

If the manifolds IM1,IM2,OM1,OM2 carry aggressive media as the processstreams it will probably be necessary to protect activated diffusionbonded joints between neighbouring plate elements from contact with theaggressive fluid in the manifold means. This can readily be done bymaking autogenous seal welds which span the joints between thepenetrated plate elements.

Referring now also to FIGS. 3 and 4, the heat exchanger plate element P1illustrated has a core structure comprising the single core sheet 202.Looking at the features of the heat exchanger plate element P1 in theorder in which they would be encountered by a stream of process fluidpassing through it, the inlet I is merely a gap between sheets 201 and203 where the core sheet 202 has been cut away by the above-mentionedmachining operation to the extent shown by outer of the concentriccircles in FIG. 3. This allows the process fluid to flow on both sidesof the core sheet 202 and hence, after traversing the inlet distributorregion DI, into all the passages P formed alternately between the coresheet 202 and the outer sheets 201,203.

The inlet I opens directly into the inlet flow distributor region DI,which is a region where the bond inhibitor was not applied to thenumerous small circular areas or dots on both the joint faces F1,F2 ofthe outer sheets (FIG. 2A). These dots are arranged in rows as shown,with each dot on a given joint face F1 being positioned midway betweeneach group of four dots on the other joint face F2. At these dots thecore sheet 202 is bonded to the outer sheets 201,203 and during thesuperplastic forming operation the core sheet 202 is expanded to thedouble cusped configuration shown in FIG. 4.

The upstanding peaks 210 and depressions 211 thus formed on both sidesof the core sheet 202 in the distributor region DI act to diffuse theflow of the process stream so that by the time it has traversed theinlet distributor DI it is distributed over the entire lateral extent ofthe core structure and enters all the passages P.

The major part of the core structure consists simply of straight linecorrugations formed in the core sheet 202. These corrugations are ofsuch a form that, in conjunction with the outer sheets 201,203,longitudinally straight flow passages P with a trapezoid shapedcross-section are defined. As shown in FIG. 4, the transition betweenthe so-called "dot core" distributor regions DI and the "line core"passage region is easily arranged.

When the heat exchange fluid reaches the ends of the passages P whichare distant from the inlet distributor DI, it encounters the outletdistributor region DO, also termed the "collector" region. This is apart of the expanded core structure which is of the same form as theinlet distributor DI, and it functions to collect the heat exchangefluid flow from over the lateral extent of the core passages P and tofeed it into the outlet manifold OM1 in a way which is distributedaround a large proportion of the manifold's periphery.

In the present embodiment, the core structure consists of a single sheet202, though it could consist of more than one sheet if a more complexcore structure is required, as shown in our copending patent applicationEP90308923.3.

The present embodiment is concerned with a simple heat exchanger plateelement in, which one process stream 101 or 102 flows through it on bothsides of the core sheet 202 and therefore through all the passages P inthe core structure. The process streams 101,102 exchange heat throughthe intimate thermal contact provided by the bonded joints betweenneighbouring plate elements. Consequently, the primary heat exchangesurfaces are the surfaces of the outer sheets 201,203, whereas thesecondary heat exchange surfaces, designated "fins", are the surfaces ofthe core sheet 202 forming the partitions between the flow passages P.

The person skilled in heat exchanger technology will realise, however,that it would be easy to arrange the inlets, outlets and the corestructure of the elements P1,P2 so as to accommodate two processstreams, one on each side of the core sheet 202, so that neighbouringflow passages P would carry different streams exchanging heat directlyacross the partitions between the passages. This would require suitablebut easily realised alteration of the form of the expanded core sheetstructure to provide the appropriate connections to the inlet and outletmanifolds.

A skilled person will also realise that alternative designs inaccordance with the invention can easily be developed to achieve heatexchange between more than two fluids. For example, for each additionalfluid, an additional inlet hole and an additional outlet hole can beprovided in the end areas of the heat exchanger elements, where thesheets are solid state diffusion bonded together with no internalstructure. The elements can then be stacked together to form a heatexchanger matrix giving heat exchange between fluids as desired. Forexample, with three fluids A, B, C, the sequence of elements within thematrix could be A/B/C/A/B/C, or A/B/B/C/A/B/B/C, or even A/B/C/A/B/B/C,to suit the heat transfer engineer.

It should be realised that the simple geometries shown for the coresheet 202 in the present drawings could readily be altered to producemore conventional finning arrangements, such as herringbone, serratedand perforated, as known in the industry.

Furthermore, for increased efficiency of heat exchange, it may bedesirable to dispense with separate passages P formed by corrugations inthe core sheet 202. Instead, the core sheet could be formed into thecusped configuration of the distributor regions throughout its wholeextent.

Moreover, it is not necessary to have the same size or form of internalstructure in all of the elements. These parameters can be chosen to suitthe fluid passing through them. Thus with, say, three fluids, the matrixcould readily consist of three different types of elements withoutunduly complicating the manufacture of the matrix.

We claim:
 1. A plate-fin type of heat exchanger for facilitatingexchange of heat between at least two process streams, comprising;amatrix of heat exchange elements arranged in side-by-side heat exchangerelationship, the heat exchange elements comprising diffusion bondedsandwich constructions, each such sandwich construction having two outersheets and a superplastically expanded core sheet structure between thetwo outer sheets, each core sheet structure providing flow passage meansfor at least one process stream, adjacent heat exchange elements beingin intimate thermal contact with each other over at least most of theareas of their side faces through bonded joints between them, andprocess stream inlet and outlet manifold means integral with the matrixfor passing the process streams through the heat exchange elements, themanifold means penetrating the matrix from side-to-side through thethicknesses of the plate elements.
 2. A heat exchanger according toclaim 1, in which the bonded joints between adjacent plate elements aremetallurgically bonded joints.
 3. A heat exchanger according to claim 2,in which the bonded joints between adjacent plate elements are activateddiffusion bonded joints.
 4. A heat exchanger according to any precedingclaim 1, in which the bonded joints are protected from contact withprocess stream fluid in the manifold means by autogenous seal weldsspanning the joints between the penetrated plate elements.
 5. A heatexchanger according to any preceding claim 1, in which thesuperplastically expanded core structures of the heat exchange elementscommunicate with the inlet and outlet manifold means through slotopenings extending peripherally of the manifold means within theexpanded core structures.
 6. A heat exchanger according to claim 5, inwhich the inlet and outlet manifold means comprise holes machinedthrough the thickness of each element to connect to the expanded corestructures.
 7. A heat exchanger according to any preceding claim 5, inwhich the inlet and outlet manifold means communicate with respectivedistributor and collector regions of the expanded core structures, thedistributor and collector regions comprising means for respectivelydistributing and collecting heat exchange fluid to and from the internalextent of the expanded core structure transversly of the generaldirection of flow therethrough.